It is known to ablate openings with lasers in substrates.
First, contact masks are utilized. A mask is placed--usually by multi-step processes--over the substrate. The mask has opening windows for ablating light where openings are desired, and is otherwise opaque where openings are not desired. Light incident on the mask at the opening windows creates the required openings. When the mask is removed--usually by an etching step--the ablated openings in the substrate remain. The wall slope is difficult to control in general since there are no ready means for differentially varying the intensity profile across the opening.
Secondly, it is known to utilize laser illuminated masks and imaging objectives to project the pattern defined by the mask onto a substrate (such as polyimide) which is ablated. In the usual case, the imaging objective takes a relatively large mask and demagnifies it at the substrate with the result that the images of the openings to be ablated have the required (increased) intensity for ablation.
Various techniques have been used for creation of imaging masks. These techniques include multi-step etching processes, which processes can be repeated to create variant mask shading at differing locations on a single mask. Relatively complex partially transmissive and partially opaque patterns can be created but in dielectric coated masks, the number of transmission levels (gray levels) is limited by the number of process steps and alignments required in its manufacture. U.S. patent application Ser. No. 4,684,436 of Burns et. al. describes this fabrication process.
An example of a prior art system containing these elements can be found in Brannon et al. U.S. Pat. No. 4,508,749 issued Apr. 2, 1985. In the above reference, the ability to make "openings of positive slope" is set forth. Other than this statement, precision control of opening shape is not suggested or set forth.
A significant drawback of the above mentioned techniques is that they lack means for varying the intensity profile across the desired openings.
Another drawback of these techniques is that no means are specified for determining the intensity or fluence profile required at the workpiece for achieving the required wall shape.
Both contact mask and imaging systems are generally inefficient since many patterns of interest generally have .ltoreq.1% of their area machined, so that in exposing the mask .gtoreq.99% of the incident light is wasted.
It has also been known for some time that a broad class of industrially useful polymer films are ablated by the action of excimer laser light and that this ablation occurs at a certain threshold fluence J.sub.th. That is, if the fluence, J, incident on the surface satisfies J&lt;J.sub.th then there is no significant ablation while if J&gt;J.sub.th there is significant ablation. "Ultraviolet Laser Ablation of Organic Polymers" by Srinivasan et. al. in Chemical Reviews, vol. 89, pg. 1303 summarizes much of the known science and phenomenology of polymer ablation. These observations have not previously been exploited for the purpose of determining desired wall profiles.
Stable structures in polyimide have been noted in the past. "Cone" structures were produced by Dyer et. al. in "Development and origin of conical structures on XeCl laser ablated polyimide", published in Applied Physics Letters, Vol. 49, No. 8, pg. 453 (1986). Particle contaminants on the surface or distributed within the volume of a polymer film produced stable conical structures when irradiated by light energy sufficient to cause ablation. These structures are stable in the sense that after a certain number of shots required for their formation, the resulting structure is immune to changes by further laser shots. According to Dyer et al., cones begin as either a low spot or null in the incident intensity pattern or debris obstructing the passage of light to a point on the polymer surface. After they are formed, their subsequent immunity to change by further laser shots is the result of their increased area over that of a flat surface and Fresnel reflectivity from the resulting sloped wall. It is important to note that there was no attempt to locally profile ablating light or control its local intensity in producing these "cones." Simply stated, the cones formed randomly in accordance with particulate dispersion and overall laser intensity applied.